A power supply, which is connected to the substrate potential of a vacuum chamber used for plasma surface treatment (delivering the bias voltage), should be a controlled voltage source. The voltage at the output of the power supply follows the voltage set-point, and is independent from the current, which is drawn. This current depends on the plasma and substrate properties, which may change in time during the treatment process. The velocity and amplitude of these changes may vary significantly, and depend on the geometry and movement (rotation) of the substrate, as well as the momentary amount of ions in the plasma chamber, which changes rapidly especially when a high power impulse magnetron is used. In order to keep the voltage value independent from the load current, a capacitor can be used.